As you must be aware by now that Windows 7 SP1 is available to download. After you apply the Windows 7 SP1 to your system and you will notice a drastic loss of hard drive space in your computer. Every Service pack installation is installed with an option to un-install in future if incompatibility or stability issue occur after the system has been updated.
This feature of being able to un-install the Service Pack 1 in future stores files in the hard drive which takes lot of system's hard drive space. You can free up this space by using dism command. It is strongly recommend to backup your system before you do so and also test your system for couple of days after the SP1 has been applied to make sure that you don't need to un-install the SP1 for any compatibility or stability issue.
Steps to remove Windows 7 SP1 backups to free up hard disk space:
1. Open the Command Prompt by selecting All Programs -> Accessories; right-click on Command Prompt and the select "Run as Administrator".
2. Type this command in the command prompt: dism /online /cleanup-image /spsuperseded
3. After a few minutes, you will see the The Operation completed successfully message.
Now, you have successfully reclaimed the disk space occupied by SP1 un-install feature.
Disclaimer: I assume you are sure of what you are doing and I take no responsibility whatsoever. Although for further study, refer to "WARNING about removing the SP1 Beta or RC uninstall Information to free up disk space!" thread.